The single phase BLDCM is widely used in low starting torque and small power fields such as pumps, blowers and cooling fans. The single phase BLDCM requires a suitable current commutation signal synchronized with the rotor position for proper operation. In most applications, a Hall-effect position sensor is used to detect the rotor position and control the motor. However, the Hall sensor itself increases the size of the motor system and the manufacturing costs. Besides, the Hall sensor decreases the capability of the system against the environment variation, e.g., the temperature variation. Many Hall-less BLDCM drives that detect the rotor position have been introduced (e.g., U.S. Pat. No. 5,986,419).
The single phase motors have null-points in their torque waveforms, which make them difficult to start sometimes. To overcome this problem, an asymmetrical air gap is employed to introduce a reluctance torque component. FIGS. 4(a) to 4(d) shows some typical models of this kind. A paper, S. Bentouati, Z. Q. Zhu and D. Howe, “Permanent Magnet Brushless DC Motors For Consumer Products”, The European Network for Permanent Magnet Devices Report (<http://mag-net.ee.umist.ac.uk/reports/P11/p11.html>), has proved that tapered-air gap is the best configuration among the above-mentioned models according to cogging torque peak value. The motors involved in the present invention adopt this kind of asymmetrical air gap to overcome their starting null-points.
Most of these applications require the motor to rotate in one predetermined direction. But the direction control becomes very difficult when the system is running in sensorless mode, more clearly in Hall-less mode, because the controller cannot determine the rotor's initial position. During the motor's starting procedure, the controller should solve this problem firstly.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicants finally conceived a method for starting a single phase BLDCM having an asymmetrical air gap. This invention is mainly about how to solve the above-mentioned problem. A novel method is introduced to make the motor start up from standstill to rotating in a preferred direction.